The invention relates to a paper machine fabric comprising at least two separate layers formed using at least two separate yarn systems: a yarn system constituting the paper side and comprising machine direction and cross machine direction yarns and a yarn system constituting the machine side and comprising machine direction and cross machine direction yarns, the yarn systems being arranged to form independent structures in the machine and cross machine directions of the fabric and the structures being bound together by means of binder yarns, a binder yarn being arranged to form part of the weave of a layer on the paper side surface and arranged to be interwoven with a layer of the machine side by being interwoven under at least one yarn in the machine side layer.
Conventional triple layer paper machine fabrics and structures bound with a binder yarn pair are known in the field. Conventional triple layer paper machine fabrics comprise two separate layers: a paper side layer and a machine side layer. The paper side layer and the machine side layer are interconnected mainly by means of a binder weft, which serves as a binder yarn. Binding with a binder yarn usually takes place at every fourth top and bottom yarn pairs, i.e. relatively seldom. On the topside, the binding takes place over one top warp and on the bottom side, under one bottom warp. The binder yarn does not contribute to the forming of the paper side surface, but only to the binding of the layers. Swedish patent 420,852 describes the technology.
U.S. Pat. Nos. Publications 4,501,303, 5,967,195 and 5,826,627, for instance, describe techniques employed for binding structures using a binder yarn pair. In the structures bound using a binder yarn pair, instead of the binder yarn, it is the binder yarn pair responsible for binding the layers. A binder yarn pair comprises two adjacent binder yarns, one of the binder yarns establishing the paper side surface weave and the other simultaneously binding a paper side layer and a machine side layer together under one machine side bottom warp and vice versa. The path of the binder yarn pair on the paper side surface establish a weft path similar to the top weft.
Typically, in conventional triple layer paper machine fabrics and in structures bound with a binder yarn pair, the diameter of the top warp is distinctly smaller than the bottom warp. As large a difference in the diameter as top warp 0.13 mm and bottom warp 0.21 mm is generally used. In these structures, each top warp in the paper side layer is bound in the same way to the top wefts according to the weave repeat interruption on the paper side, and each bottom warp in the machine side layer is bound in the same way to the bottom wefts according to the weave repeat interruption on the machine side.
Both conventional triple layer paper machine fabrics and structures bound with a binder yarn pair usually employ as many top warps as bottom warps, i.e. warp ratio is 1:1. Since the number of top warps is equal to that of bottom warps, weft density cannot be raised sufficiently. Thick bottom warps and the relatively high density of the top warps also complicate raising weft density. When weft density remains low, the openings on the paper side surface are in the shape of a rectangle standing on the short side, i.e. the long side is parallel to the machine direction. When a paper web is formed, paper fibers are oriented in the machine direction. In other words, the paper fiber and the openings in the paper machine fabric are parallel, resulting in a poor support for the paper fiber.
In structures bound with a binder yarn pair, the yarns in the binder yarn pair cross at a point where one binder yarn descends in the fabric from the paper side in order to bind the layers, while the other binder ascends in the fabric to form the surface of the paper side. The top weft positioned at both sides of the intersection presses the top warp yarns at the intersection downwards and, simultaneously, both yarns of the binder yarn pair descend into the fabric, not supporting the top warp yarns from below. Consequently, the intersections remain on a lower plane than the surface, which may cause marking.
Abrasion of a binder yarn inside the fabric causes often ‘innerside wear’ in conventional triple layer paper machine fabrics. The abrasion causes the fabric to lose its original thickness on the inner side of the fabric, while the binder yarn, however, retains its original length, making the binder yarn project from the surface of the wire, subjecting the paper web to the risk of marking. Strong innerside wear may cause the binder yarns to break and the layers to become delaminated from each other.
Innerside wear may also be found in structures bound with a binder yarn pair. A binder yarn pair formed from thin binder yarns does not bind the thick bottom warps sufficiently tightly, resulting in a loose structure and causing the risk of innerside wear. The use of thick bottom warps results in a thick fabric, and the loose binding further thickens the fabric. This causes a large void volume in the paper machine fabric, resulting in water carrying of the paper machine fabric in the paper machine, and splashing may occur in some fast paper machines. Splashing occurs in a paper machine at the point where the top wire turns to the return cycle, and in the worst case the splashing causes weakening of the quality of the paper web. Since a thick paper machine fabric impairs the effect of vacuum and dewatering elements compared with a thin paper machine fabric, the dry matter content in the paper is reduced. Another reason for a low dry matter content is a large void volume, which increases ‘rewetting’. In rewetting, the water removed from the paper web to the wire is absorbed back to the paper web in the wire section after the last dewatering elements before the press section. Because the paper web is wetter when entering the press section, breaks increase and, on the other hand, the steam consumption in the paper machine increases. Both factors significantly raise the costs at a paper machine.
A thick bottom warp also causes a high bending of the paper machine fabric in the machine direction, which is a problem in papermaking and dewatering. In the machine direction, a stiff paper machine fabric does not follow to the dewatering equipment, resulting in less turbulence and impaired dewatering and paper web formation. Herein, turbulence refers to whirling and mixing of the dewatering equipment caused by the paper web.